1. Field of the Invention
The present invention relates to a flat coil component, a characteristic adjusting method of the flat coil component, an ID tag, and an characteristic adjusting method of the ID tag.
2. Description of the Related Art
Recently, in various processings such as individual verification, merchandise management, and distribution process, ID tags (also known as IC cards) have come to be used. In particular, non-contact type ID tags are used widely.
Non-contact type ID tags are operated either by internal batteries, or by energy of radio wave or magnetic flux from a reading device of ID tags, and in the latter case, for example, necessary energy is obtained by utilizing the function of a flat coil component or an LC resonance circuit, using a flat coil component as an antenna element (Japanese Patent Laid-open No. 11-259615, Japanese Patent Publication No. 2814477).
FIG. 8 explains an example of antenna pattern in a conventional non-contact type ID tag 10.
In a plan in FIG. 8, the ID tag 10 has a flat coil 11, a jumper 12, an IC chip 13, and an IC connection wire 14 on an insulating substrate 15.
The flat coil 11 has a function of an antenna, and in FIG. 8 a conductive pattern made of conductive material such as aluminum foil, copper foil or silver paste is spirally wound and formed in a rectangular shape. In the case of FIG. 8, the end portion positioned at the inside of the spiral form of the flat coil 11 is directly connected to one terminal of the IC chip 13. The end portion positioned at the outside of the spiral form of the flat coil 11 is connected to one end of the jumper 12.
The jumper 12 is designed to transmit the electric characteristic (for example, potential) at the outside end portion of the flat coil 11 to the inside of the spiral form of the flat coil 11, and it is insulated from each loop of the flat coil 11. The jumper 12 is composed of a conductive layer 12A and an insulating layer 12B as described below (see FIG. 9).
The IC connection wire 14 is connected to the inside end portion of the jumper 12 and the other terminal of the IC chip 13.
In the IC chip 13, the unique code of the ID tag and other data are stored, and the stored data is transmitted by receiving a reading signal from a reading device not shown (also known as ID tag reader). The energy necessary for such transmitting and receiving process by the IC chip 13 is obtained mainly from the flat coil 11.
FIG. 9 is a magnified sectional view of intersection of a certain loop of the flat coil 11 and the jumper 12.
In FIG. 9, on the insulating substrate 15 made of plastic resin such as polyethylene terephthalate or polycarbonate, the flat coil 11 made of copper foil or the like is provided, and the jumper 12 is provided on the flat coil 11. The jumper 12 is, in the case of FIG. 9, composed of an insulating layer 12B made of an insulating resist resin responsible for the insulating function from the flat coil 11, and a conductive layer 12A of silver paste or the like responsible for the conductive function.
Thus, at the intersection of a certain loop of the flat coil 11 and the jumper 12, since the insulating layer 12B is enclosed by two conductive layers (flat coil 11 and conductive layer 12A), it functions as a capacitor.
The capacitance component CB by this capacitor is determined by the distance between two conductive layers (flat coil 11 and conductive layer 12A), in other words, the thickness D of the insulating layer 12B, the specific inductive capacity εr of the insulting layer 12B, and the overlapping area S of the loop of the flat coil 11 and conductive layer 12A as shown in FIG. 10, and it can be expressed in formula (1), where ε0 is the dielectric constant of air.CB=ε0·εr·S/D   (1)
In the case of FIG. 8, since the number of intersections of the flat coil 11 and jumper 12 is seven, it is equivalent that there are seven capacitors, and the combined capacitance of the capacitor components of these seven capacitors is the capacitance C of the jumper 12.
Herein, supposing the capacitance of the IC chip 13 to be Cic, the resonance frequency F in FIG. 8 can be expressed in formula (2), where L is the inductor value due to the flat coil 11.F=1/2π√{L·(Cic+C)}  (2)
That is, by making use of the LC resonance, the energy necessary for the transmitting and receiving process by the IC chip 13 is obtained. The resonance frequency F is the exchange frequency between the ID tag 10 and the reading device (not shown).
In the conventional non-contact type ID tag, however, fluctuations of resonance frequency are significant in individual products.
The jumper 12 occupies a very small area as seen from the entire substrate area, and therefore, generally, the insulating layer 12B is first formed by applying the insulating resist resin, and then the conductive layer 12A is formed by silk printing of silver paste or the like.
However, when the insulating layer 12B is formed by coating, the precision of the thickness (that is, D) is prone to decline (fluctuations are large).
When the precision of the distance D is lower (fluctuations are larger), the precision of the capacitance of the jumper 12 having its effect is forced to be lower (product fluctuations are increased), and the resonance frequency F is deviated from the desired frequency in many ID tags 10.
In the case of an ID tag 10 large in error of the resonance frequency from the desired resonance frequency F, the electric power receiving efficiency is lowered, and the communication distance with the reading device becomes shorter.
The invention is devised in the light of such problems, and it is an object thereof to present a flat coil component, characteristic adjusting method of flat coil component, ID tag, and characteristic adjusting method of ID tag, capable of suppressing product fluctuations of desired characteristics.